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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>

    <script>

        var res = alert(666) ;   //alert没有返回值
        //    document.write(res) ;    // undefined


        function fn() {
            var arr = [] ;
            for(var i = 0 ; i <= 9 ; i++) {
                arr.push(i) ;
            }
            // return console.log(arr) ;
            // var res = console.log(arr) ;
            // return res
            return arr 
        }

        // console.log(fn().length) ;




        console.log(arr2.length) ;
        var arr2 = [1,2,3] ;

    </script>
    
</body>
</html>